As is well known, for example, a power source circuit, which is used by being incorporated into, for example, an electric product and a mechanical product, is mounted with a transformer, a step-up transformer, a rectifier, and the like. The transformer and the like include various coil components each formed of a magnetic core and a winding as main parts, such as a choke coil, a power inductor, and a reactor. In order to respond to a request for low power consumption with respect to the electric product and the mechanical product on the background of increasing consciousness of energy saving in recent years, there is a demand for improvements in magnetic characteristics of the magnetic core to be used frequently in the power source circuit and a reduction in magnetic loss of the magnetic core. Further, in recent years, with increasing consciousness of a global warming issue, there has been an increasing demand for a hybrid electric vehicle (HEV), which can suppress consumption of fossil fuel, and an electric vehicle (EV), which does not directly consume fossil fuel. Running performance and the like of the HEV and the EV depend on performance of a motor. Therefore, there is also a demand for improvements in magnetic characteristics and reduction in magnetic loss of a magnetic core (a stator core or a rotor core) to be incorporated into various motors.
Hitherto, as the magnetic core, a so-called laminated magnetic core in which steel plates (magnetic steel plates) whose surface is covered with an insulating coating film are laminated through intermediation of an adhesive layer has been widely used. However, such laminated magnetic core has a low degree of freedom of a shape and is difficult to respond to a request for miniaturization and a complicated shape. Thus, there has been developed a so-called powder magnetic core obtained by subjecting a soft magnetic metal powder (metal powder having a small coercive force and a large magnetic permeability, which is generally a metal powder containing iron as a main component) whose surface is covered with an insulating coating film to compression molding. The powder magnetic core has been mounted on various products.
Meanwhile, as one of the effective means for improving the magnetic characteristics of the magnetic core, there is given means for decreasing the coercive force of the magnetic core. This is because, when the coercive force is decreased, a magnetic permeability increases whereas a hysteresis loss (iron loss) decreases. The coercive force of the powder magnetic core depends on, for example, a particle diameter, impurity content, and strain amount of the soft magnetic metal powder forming a powder for molding into a powder magnetic core (hereinafter referred to as “powder for a magnetic core”). As one of the effective means for easily obtaining a powder magnetic core having a small coercive force, there is given means for removing a strain (crystal strain) accumulated in the soft magnetic metal powder during powder production, during compression molding into a compact, and the like. In order to properly remove the strain, it is necessary to heat the compact at a recrystallization temperature or more of the soft magnetic metal powder (metal) for a predetermined period of time. For example, in the case of molding a powder for a magnetic core including a pure iron powder and an insulating coating film covering a surface of the pure iron powder into a compact, it is necessary to heat the compact at 600° C. or more, preferably 650° C. or more, more preferably 700° C. or more. Note that, a heating temperature and heating time of the compact are appropriately adjusted depending on a purity of the soft magnetic metal powder to be used and the like.
Thus, the insulating coating film for covering the surface of the soft magnetic metal powder desirably has high heat resistance. The reason for this is as described below. When the heat resistance of the insulating coating film is insufficient, the insulating coating film is damaged, decomposed, peeled, and the like along with heating treatment, and hence the heating treatment cannot be performed at high temperature at which the strain accumulated in the soft magnetic metal powder can be removed properly. As specific examples of the insulating coating film having high heat resistance, there are known an insulating coating film having a two-layered structure formed of a high-resistance substance and a phosphate-based chemically treated coating film covering the surface of the high-resistance substance (Patent Literature 1), an insulating coating film formed of an alkoxide coating film made of an Al—Si—O-based composite oxide and a silicone resin coating film formed on the alkoxide coating film (Patent Literature 2), an insulating coating film formed of an insulating layer of at least one kind selected from an oxide, a carbonate, and a sulfate, and a silicone resin layer formed on the insulating layer (Patent Literature 3), and the like.